The substitution reaction can be schematically described as:
E -X + Y ⇄ E -Y + X
where E represents the reaction center, X the leaving group and Y the incoming group. The reverse reaction is another sustitución. In fact, they are always reversible processes that lead to equilibria more o less displaced toward one side.
The mechanism of these reactions depends very much on the E center, and can be: (Ingold nomenclature): Dissociative (D) Associative (A) or Exchange (I). The latter may approach dissociative, and would be Id, or associative and would be Ia. In this section we will discuss with general examples, these three mechanisms for the most frequent types of inrganic compounds: tetrahedral, octahedral and plane-square.